Chemical Kinetics

Deals with rates and mechanisms of chemical reactions.

Collision Theory: in order to react, particles

must effectively collide with each other.

During effective collisions, electrons shift between atoms

old bonds are broken and new ones form.

What makes a collision effective?

Particles must have: the right speed or

kinetic energythe correct orientation to

each other.

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/collis11.swf

Activation Energy: minimum amount of energy needed for a reaction to occur

http://www.kentchemistry.com/links/Kinetics/FactorsAffecting.htm

Reaction Rate: speed a reaction occurs.

Rate depends on frequency and effectiveness of collisions between reactant particles.

Measured by how fast reactants are used up or product produced.

Reaction Mechanisms: series of steps by which particles rearrange themselves to form products. Most products don’t form from a single collision

Rate Determining Step: the slowest step. Usually means that step has the highest activation energy

Honors Extra practicehttp://chemwiki.ucdavis.edu/Physical_Chemistry/Kinetics/Rate_Laws/Reaction_Mechanisms/Rate-Determining_Step

Factors Affecting Reaction RateTemperature:Increasing temp. increasesreaction rate

Faster particles move = greater frequency of collisions

Greater chance collision will be effective

More particles will have enough energy to overcome activation energy

http://www.kentchemistry.com/links/Kinetics/FactorsAffecting.htm

Factors Affecting Reaction RateConcentration: Greater conc. of reactants = greater chance of

collisionMore concentrated = less space between

particlesEx: Zn in 1M HCl versus Zn in 6M HCl

http://www.kentchemistry.com/links/Kinetics/FactorsAffecting.htm

Factors Affecting Reaction Rate

Pressure: (gases only) Compressing a gas will increase

concentration of reactant particles. Increase in concentration allows for

more collisions

Factors Affecting Reaction RateSurface Area: (of a solid reactant)Greater the surface area, the more the

reactant particles are exposed to each other for possible collisions.

Ex: steel wool vs. steel bar, block of wood vs. sawdust

Ways to increase surface area:Grind into fine powderStir or agitate to disburse particles

Factors Affecting Reaction RateNature of Reactants: Simpler reactants will react faster

than ones with many complex bonds.

Fastest to React: (aq) ionic compounds have simple ions that are disbursed in

solution

Covalents are usually slower to react.

Gases usually react faster than substance in the liquid or solid phases.

Factors Affecting Reaction RateCatalysts:

speed up the rate of a chemical reaction without being altered. (ex: enzymes)

Works by providing an alternate reaction pathway that require less energy.

Lowers the amount of activation energy needed to start reaction

http://www.kentchemistry.com/links/Kinetics/FactorsAffecting.html

Inhibitors: slow down chemical reactionsCan tie up reactants in side reactionsCan block active site in enzymatic catalystsCan provide alternate reaction pathway with

higher activation energy requirements.

Sample Regents Questions

Given the balanced equation representing a reaction:

2HCl(aq) + Na2S2O3(aq)--> S(s) + H2SO3(aq) + 2NaCl(aq)

Decreasing the concentration of Na2S2O3(aq)

decreases the rate of reaction because the

(1) activation energy decreases(2) activation energy increases(3) frequency of effective collisions decreases(4) frequency of effective collisions increases

As the temperature of a chemical reaction in the gas phase is increased, the rate of the reaction increases because

(1) fewer particle collisions occur(2) more effective particle collisions occur(3) the required activation energy increases(4) the concentration of the reactants increases

Why can an increase in temperature lead to more effective collisions between reactant particles and an increase in the rate of a chemical reaction?

(1) The activation energy of the reaction increases.(2) The activation energy of the reaction decreases.(3) The number of molecules with sufficient energy to react increases.(4) The number of molecules with sufficient energy to react decreases.

At 20.°C, a 1.2-gram sample of Mg ribbon reacts rapidly with 10.0 milliliters of 1.0 M HCl(aq). Which change in conditions would have caused the reaction to proceed more slowly?

(1) increasing the initial temperature to 25°C(2) decreasing the concentration of HCl to 0.1 M(3) using 1.2 g of powdered Mg(4) using 2.4 g of Mg ribbon

A 5.0-gram sample of zinc and a 50.-milliliter sample of hydrochloric acid are used in a chemical reaction. Which combination of these samples has the fastest reaction rate?

(1) a zinc strip and 1.0 M HCl(aq)

(2) a zinc strip and 3.0 M HCl(aq)

(3) zinc powder and 1.0 M HCl(aq)

(4) zinc powder and 3.0 M HCl(aq)

Given the balanced equation representing a reaction:

Zn(s) + 2HCl(aq)==>H2(g) + ZnCl2(aq)

Which set of reaction conditions produces H2(g) at the fastest rate?

(1) 1.0-g lump of Zn(s) in 50. mL of 0.5 M HCl at 20.oC(2) 1.0-g lump of Zn(s) in 50. mL of 0.5 M HCl at 30.oC(3) 1.0 g powdered Zn(s) in 50. mL of 1.0 M HCl at 20.oC(4) 1.0 g powdered Zn(s) in 50. mL of 1.0 M HCl at 30.oC

Thermodynamics

Study of the energy involved in chemical reactions.

Particles must have a certain energy to react

During collisions KE (of motion) is converted to PE (stored energy in chemical bonds)

PE Diagrams

Show how PE changes during a chemical rxn.

PE ReactantsPE Products

Activation Energy: energy needed to startenergy required to form

“activated complex”

Activated Complex: intermediate between reactant and productFound at top of energy hillMust be formed for rxn to occurUnstable and short lived (very high in energy)Contain bonds in process of being broken/formed

Heat of Reaction (∆H)Overall net change in energy (enthalpy) for reactionDifference between PE of products and reactants

∆H = PE products – PE reactants

What is ∆H for this reaction?

Endothermic vs. Exothermic

Exothermic: (∆H negative) Products have less

energy than reactants Energy given off

Endothermic: (∆H positive) Products have more

energy than reactants Energy taken in

Usually requires more activation energy than exothermic reactions

http://www.youtube.com/watch?v=VbIaK6PLrRM&feature=player_embedded

Determine the following:PE reactantsPE ProductsPE activated complexForward activation

energyReverse activation

energy∆H for forward rxnExo or endo?

http://www.kentchemistry.com/links/Kinetics/PEDiagrams.htm

Affects of a CatalystCatalysts lower:

activation energy for both the forward and

reverse reactionsEnergy of the activated complex

How much energy was saved by using a catalyst?

Catalysts Don’t Change ∆H

They do NOT EFFECT:PE of reactants or products.∆H (overall heat lost or gained)

Which interval on this diagram represents the difference between the potential energy of the products and the potential energy of the reactants?   

(1) 1     (2) 2    (3) 3      (4) 4

Which statement is true about energy in this reaction?

(1)The reaction is exothermic because it releases heat.     

(2)The reaction is exothermic because it absorbs heat.

(3)The reaction is endothermic because it releases heat.

(4)The reaction is endothermic because it absorbs heat.

Which statement best describes how a catalyst increases the rate of a reaction?

(1) The catalyst provides an alternate reaction pathway with a higher activation energy.

(2) The catalyst provides an alternate reaction pathway with a lower activation energy.

(3) The catalyst provides the same reaction pathway with a higher activation energy.

(4) The catalyst provides the same reaction pathway with a lower activation energy.

Which statement correctly describes the energy changes that occur in the forward reaction?

(1) The activation energy is 10. kJ and the reaction is endothermic.

(2) The activation energy is 10. kJ and the reaction is exothermic.

(3) The activation energy is 50. kJ and the reaction is endothermic.

(4) The activation energy is 50. kJ and the reaction is exothermic.

In a chemical reaction, the difference between the potential energy of the products and the potential energy of the reactants is defined as the

(1) activation energy

(2) ionization energy

(3) heat of reaction

(4) heat of vaporization

Which information about a chemical reaction is provided by a potential energy diagram?

(1) the oxidation states of the reactants and products

(2) the average kinetic energy of the reactants and products

(3) the change in solubility of the reacting substances

(4) the energy released or absorbed during the reaction

Which statement correctly describes an endothermic chemical reaction?

(1) The products have higher potential energy than the reactants, and the ΔH is negative.

(2) The products have higher potential energy than the reactants, and the ΔH is positive.

(3) The products have lower potential energy than the reactants, and the ΔH is negative.

(4) The products have lower potential energy than the reactants, and the ΔH is positive.

Given the reaction:

S(s) + O2(g) → SO2(g) + energy 

Which diagram best represents the potential energy

changes for this reaction?

Reference Table IHeats of Reaction Table (∆H ):

Shows various reactions and whether they are endothermic or exothermic.

- ∆H exo+ ∆H endo

The energy involved is for the relative number of moles indicated in the balanced equation

Using Table ITable shows several types of reactions:

Top 6 reactions are combustions (- ∆H )Middle reactions are synthesis

∆H values are showing ∆H of “formation” of the compound from its elements

Bottom reactions show dissolving of various ionic salts (some are exo, some endo)

Last reaction is neutralization

Using Table I

Ex: 2CO(g) + O2(g) → 2 CO2 + 566 KJ

Exothermic reaction ∆H = -566.8KJThe reverse of this reaction would be

endothermic ∆H = +566 KJ (←)

If just one mole of CO used, half the energy would be produced ∆H = - 283KJ

Given the reaction:

2 H2(g) + O2(g) --> 2 H2O(l) + 571.6 kJ

What is the approximate ΔH for the

formation of 1 mole of H2O(l)?

(1) -285.8 kJ        

(2) +285.8 kJ  

(3) -571.6 kJ    

(4) +571.6 kJ

Chemical cold packs are often usedto reduce swelling after an athleticinjury. The diagram represents thepotential energy changes when acold pack is activated.

Which lettered interval on thediagram represents the potentialenergy of the products? 

Which lettered interval on thediagram represents the heat ofreaction?

Identify a reactant listed inReference Table I that could bemixed with water for use in achemical cold pack.

According to Table I, which salt releases

energy as it dissolves? 

(1) KNO3       

(2) LiBr

(3) NH4NO3 

(4) NaCl

Given the reaction:

CH4(g) + 2 O2(g) --> 2 H2O(l) + CO2(g)

What is the overall result when CH4(g) burns

according to this reaction?

(1) Energy is absorbed and ΔH is negative.      

(2) Energy is released and ΔH is negative.

(3) Energy is absorbed and ΔH is positive.  

(4) Energy is released and ΔH is positive.

According to Table I, which potential energy diagram best represents the reaction that forms H2O(l) from its elements?

(1)(2)(3)(4)